Enhancing Solids Destruction from Anaerobic Municipal Digesters

The purpose of this study was to compare temperature phased and temperature staged anaerobic digestion to conventional mesophilic anaerobic digestion in terms of solids destruction, biomass activity, and pathogen inactivation. Previous studies have established that temperature phased anaerobic digestion (TPAD) may produce more methane, destroy volatile solids, and inactivate pathogenic microorganisms more completely than comparable conventional mesophilic digestion. Also previous studies showed that batch-staged temperature anaerobic digestion (B-STAD) and anaerobic digestion elutriated phased treatment (ADEPT) can produce lower volatile fatty acids (VFA) concentrations in the effluent. The study reported herein also provides information on nutrient addition effects on effluent VFA concentrations and biomass activity. The conventional mesophilic, TPAD, B-STAD, and ADEPT pilot digesters operated for this study were each fed once a day and, therefore, received a daily slug dose of feed. This had the advantage of holding all particles of sludge for 24 hours at 55°C. However, the daily slug feeding typically resulted in elevated volatile fatty acid concentrations, especially in the thermophilic digester. Each system had a total active volume of 24 liters and was operated at a 15 day hydraulic retention time (HRT). The single-phase conventional anaerobic digestion system was operated under mesophilic conditions. The TPAD system consisted of an 8-liter thermophilic reactor followed by a 16-liter mesophilic reactor. The B-STAD system consisted of the same reactors used in the TPAD system but configured to recycle effluent from the mesophilic reactor to the thermophilic reactor. The ADEPT system consisted of a 16-liter thermophilic reactor followed by an 8-liter mesophilic reactor. Reactors used in the ADEPT system were not mixed. Each system was fed a municipal sludge blend of 70% primary sludge and 30% waste activated sludge from the City of Janesville Wastewater Treatment Plant. The advantages of the TPAD system over the conventional digestion system were the production of Class A biosolids and higher generally biomass activity. However, solids destruction results were not statistically different. Daily addition of alkalinity or trace nutrients to the: TPAD system was required to buffer the pH above 6.8 due to the high VFA concentrations in the thermophilic reactor. Alkalinity addition to the TPAD was substituted by nutrient medium addition which resulted in a significant decrease in the VFA concentrations in the TPAD effluent. The recycling of thermophilic biomass to the thermophilic digester as performed for the B-STAD system showed the advantages of the TPAD system along with the elimination of alkalinity or nutrient addition to maintain thermophilic pH above 6.8